There are currently no approved antiviral therapies that could be rapidly deployed in the event of a bioterrorism attack using a poxvirus or the unexpected spread of an indigenous agent like monkey poxvirus to other parts of the world. Initial efforts in drug development for poxvirus infections have focused on the identification of compounds that are already approved for use in some other infection. The best candidate currently is cidofovir, which is approved for treatment of certain cytomegalovirus infections, is a potent inhibitor of poxvirus replication in vitro and has been shown to be very effective against both vaccinia virus (VV) and cowpox virus (CV) infections in animal model infections. Its nephrotoxicity and poor oral availability, however, limit its usefulness. Therefore, there is a continued need to develop new and better modes of therapy for Poxvirus Infection. The specific aims of this SBIR Phase I project are to: 1) Synthesize new series of pyrimidine nucleosides (about 30), which are designed on the basis of rationale described in the sections B. C and D; 2) Evaluate the antiviral efficacy of these nucleosides against VV and CV by plaque reduction assay in tissue culture in collaboration with Prof. Earl R. Kern of the University of Alabama at Birmingham; and 3) Determine the efficacy of lead compounds in animal model infections of VV and CV, also in collaboration with Professor Kern. In addition, the in vitro and in vivo toxicity of the compounds will be determined in-house as well as in collaboration with Dr. Kern. We plan to synthesize 31 novel pyrimidine nucleosides for antiviral evaluation in tissue culture systems. We estimate, based on our preliminary results, that three of four compounds will have sufficient activity in the animal model infections and select one or two candidates for further development in Phase II. In Phase II, we plan to further investigate those active nucleosides selected in Phase I for their detailed preclinical pharmacological and toxicological features in small animals and dogs for IND application.